Removed pytest warning from deleting from callback_store

[RobertoRoos]

Resolves #493:

• pytest.PytestUnraisableExceptionWarning for a KeyError in tests #493

[coveralls]

Pull Request Test Coverage Report for Build 21254060182

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on bugfix/del-notification at 93.389%

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Totals
Change from base Build 20993886363:	93.4%
Covered Lines:	10708
Relevant Lines:	11466

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💛 - Coveralls

[stlehmann]

Not sure about it but generally I like the idea of getting an exception if something unintended (e.g. KeyError) happens. It might be just to log a Warning. Wouldn't want to let it slip unnoticed.